You can cancel a classic game that has already started by selecting the box with a white flag in the lower left corner of the screen. Once you press it, the game will ask you if you want to quit and you must press "Resign" to end the game.
Keep in mind that this will negatively impact your stats, as the game will be recorded as lost. For this reason, if you receive a request to play a game that you do not wish to participate in, we recommend that you reject it before starting to play. This way, your stats will not be affected.